1. Field of the Invention
The present invention relates to an image forming apparatus which includes a medium transporting means which transports an image recording medium while holding the medium thereon, and forms a multi-color image in such a way that color toner images are formed in a plural number of toner image forming units and successively transferred onto an image recording medium in a superposing manner.
2. Discussion of the Prior Art
An electrophotographic image forming apparatus of the type in which toner images of different colors are successively transferred onto an image recording medium in a superposing manner, to thereby form a multi-color image, is well known. The image forming apparatus includes a medium transporting means which transports an image recording medium while sucking the medium, and a plural number of photoreceptors disposed along the medium transporting means. Further, a charger, a scan optical system, a developing unit, and the like are disposed around each of the photoreceptors. An image recording medium is fed onto the medium transporting means and attractively held by the same. The image recording medium, carried by the medium transporting means, moves forward successively by the photoreceptors. Through the movement of the image recording medium, toner images are transferred onto the image recording medium in a superposing manner, from the photoreceptors.
The following means to cause the medium transporting means to attract the image recording medium to the medium transporting means per se is widely used for the image forming apparatus of this type.
The medium transporting means taking the form of an endless belt or a drum is known. The outer surface of the medium transporting means is constructed with a dielectric sheet made of polyvinylidene fluoride, for example. A medium attracting charger as charge applying means is provided on the inner side of the medium transporting means. Outside the medium transporting means, a tubular electrode roller, electrically earthed, is disposed in opposition to the paper attracting charger with respect to the medium transporting means.
In the medium attracting means, the medium attracting charger supplies charge to the medium transporting means. An electric field is formed in a medium attracting region where the medium transporting means is opposed to the electrode roller. Charge of which the polarity is opposite to that of the charge of the medium transporting means is induced into the electrode roller. The image recording medium is fed into the medium attracting region while being guided by a guide member. The image recording medium, which is brought into the electrode roller, receives the electrode roller, and is electrostatically attracted to the medium transporting means being charged in the polarity opposite to that of the charge of the image recording medium.
In the image forming apparatus, in transferring the toner images onto the image recording medium, it is necessary to register these toner images with high precision. The image forming apparatus in which reference images are directly transferred onto the surface of the medium transporting means, and the transferring positions of the toner images are adjusted using the transferred reference images is known. Means to adjust a density of each toner image in which a reference pattern image is formed and directly transferred onto the medium transporting means, and a detecting means, e.g., a CCD sensor, detects a density of the toner image, is also known. After the positions and the densities of such toner images are detected, the toner images are removed by cleaning means including a blade, a roller and the like, located downstream of the image forming stage. Also during the process of forming images, toner charged in low level and toner charged in the opposite polarity are transferred onto a nonimage area on each photoreceptor, and the transferred toner are occasionally transferred onto the surface of the medium transporting means. This type of the toner is also removed by the cleaning means.
These conventional image forming apparatuses have the following disadvantages.
The toner attached to the surface of the medium transporting means is frequently incompletely removed by the cleaning means, and hence left thereon after it passes the cleaning means. Particularly, as disclosed in Japanese Patent Unexamined Publication No. Sho 62-51530, at the joint portion of the dielectric sheet where it is jointed, it is very difficult to completely remove the toner attached thereto. Accordingly, insufficient removal of toner is frequent. With the circulative turn of the medium transporting means, the toner left on the joint portion reaches the medium attracting region, and sometimes is transferred to the electrode roller. Let use consider a case where a voltage for attracting the image recording medium is applied between the electrode of the medium attracting charger and the electrode roller when the image recording medium is present in the medium attracting region, and the same voltage is also applied when the image recording medium has gone by the medium attracting region and is not present there. In this case, when the image recording medium is not present in the medium attracting region, an intensive electric field generated therein causes the toner left on the medium transporting means to go to the electrode roller even when the toner is charged in the direction opposite to the direction of the magnetic field. After one turn of the electrode roller, the toner thus transferred to the electrode roller is transferred again to the surface of the image recording medium when it reaches the medium attracting region. The toner transferred to the medium surface causes a defective image.
One of possible ways to prevent the residual toner from attaching to the electrode roller is to bring the electrode roller into contact with the medium transporting means only when the image recording medium is attracted, and to retract it from the medium transporting means when it is not attracted. When the electrode roller is brought into contact with the medium transporting means and is retracted therefrom, a load of the medium transporting means varies. The load variation leads to deterioration of the image quality.
There is a proposal to solve the unsatisfactory removal of toner. In the proposal, to smooth the surface of the medium transporting means, the joint portion on the medium transporting means is covered with a tape. The proposal is unsatisfactory in durability and hence needs much maintenance, however.